Silicones or polyorganosiloxanes are materials which have suitable properties for applications in various fields such as biomedical materials, electronic devices, etc. This has led to research and development in silicones with a view to improving the properties and processes involved. Prior to use, the silicones typically undergo chemical crosslinking to improve their strength to a suitable level for applications. Either thermal or light curing is generally used, depending mainly on functional groups in silicone structure and applications. For example, silicone coated on electronic devices is typically subjected to light curing since thermal curing may damage the items, and silicone dental sealants is generally subjected to light curing. At present, the light curing is more widely used.
One well-known reaction for silicone curing is a hydrosilylation reaction, which is a reaction between a silanichydride (Si—H) bond in silicone molecule chain and a carbon-carbon double bond in polyorganosiloxanes using a platinum catalyst. In this method, heat is normally applied to accelerate the curing.
Polyorganosiloxanes having a functional group susceptible to free radical reaction, e.g., acrylate or methacrylate group, can undergo thermal and/or light curing, when thermal- and/or photo-initiators are used, respectively. Several synthetic methods of preparing acrylate-/methacrylate-functionalized polyorganosiloxanes/silicones are known. U.S. Pat. No. 6,548,568 discloses the hydrosilylation reaction of a silanichydride bond in polysiloxane chain with a carbon-carbon double bond in allyl glycidyl ether using a platinum catalyst. Polysiloxane in this reaction was prepared by ring-opening polymerization of octamethylcyclotetrasiloxane (D4) with bis-trimethylsilyl methyl-hydrogen polysiloxane (MD′50M) using hexamethyldisiloxane (D2) as a chain terminator. After the hydrosilylation reaction, the obtained product having epoxy function can further react with an acrylate monomer. The silicone in the final stage would have acrylate groups in the structure, which are used as UV light curing sites; and methyl groups at the polymer chain ends (using D2 as a chain terminator).
U.S. Pat. No. 7,009,024 discloses a preparation of aromatic-based siloxane macromonomers, which provide UV light curable silicones. The preparation of the siloxane macromonomers includes two steps—the first step is a synthesis of hydride functionalized cyclic siloxanes, e.g., by reaction between tetramethylcyclotetrasiloxane (D4H) and 1,3-bis(4-methacryloyloxybutyl)tetramethyldisiloxane (M2, methacrylate-capped disiloxane), giving a product having silanichydride bonds and methacrylate groups at the molecule chain ends; the second step is a hydrosilylation of silanichydride bonds in the prepared product from the first step with aromatic functionalized allyl compounds, yielding a silicone macromonomer with aromatic side chains. In the light curing of the macromonomer, methacrylate monomer is also added. The obtained product can be used for ophthalmic devices such as intraocular tens.
WO 03/002635 discloses a preparation method of acrylate groups at both polymer chain ends using a hydrosilylation reaction of polydimethylsiloxane, dihydride terminated, with carbon-carbon double bond in allyl epoxide, followed by an epoxide ring opening reaction of the obtained product with polyols. The resulting product is then reacted with unsaturated carboxylic acid, e.g., methacrylic acid, giving acrylate groups at both polymer chain ends in the final step.
A disadvantage of hydrosilylation reaction of silanichydride bonds in polysiloxane or siloxane macromonomer is that it is difficult to control the degree of substitution due to the high molecular weight (high viscosity) of the starting substance and a vigorous reaction condition: e.g., high temperature, prolonged reaction time, and a large amount of catalyst is required compared to hydrosilylation reaction of silicone monomer.
R. A. Ortiz et al. (“Synthesis of hybrid methacrylate-silicone-cyclohexanepoxide monomers and the study of their UV induced polymerization”, Progress in Organic Coatings, 57, (2006), 159-164) discloses a preparation of hybrid silicone monomer having a methacrylate group and epoxy ring in the structure using a hydrosilylation reaction of linear silicone hydride monomer—started by a hydrosilylation reaction between a silanichydride bond in the monomer with carbon-carbon double bond in a methacrylate-containing reagent, e.g., allyl methacrylate. The prepared hybrid silicone monomer is thermal and light curable.
Y. Kang et al. (“Ionic Conductivity and Electrochemical Properties of Cross-linked Solid Polymer Electrolyte Using Star-shaped Siloxane Acrylate”, Journal of Power Sources, 165, (2007), 92-96) discloses a synthesis of star-shaped siloxane acrylate by the hydrosilylation reaction of tetramethylcyclotetrasiloxane (D4H) with poly(ethylene glycol)monoallyl ether, giving hydroxy-containing siloxane monomer undergoing acryloylation in the next step. The final product is siloxane monomer having acrylate groups at the chain ends, which is employed as a crosslinker by thermal curing with a thermal initiator (benzoyl peroxide (BPO) was used).
A synthesis method of siloxanes with various functional groups at the chain ends is disclosed in M. Cazacu et al. (“Synthesis of Functional Telechelic Polydimethylsiloxane by Ion-exchangers Catalysis”, European Polymer Journal, 35 (1999), 1629-1635) which discloses a preparation of telechelic siloxane oligomers with various functional groups, such as vinyl (—CH═CH2), aminopropyl (—(CH2)3—NH2), methacryloxymethyl (—CH2—O—OC(CH3)C═CH2), or carboxypropyl (—(CH2)3—COOH) at the polymer chain ends, using a ring-opening polymerization of octamethylcyclotetrasiloxane in disiloxanes with various described functional groups at both molecule ends (as a chain terminator of synthesized siloxane oligomers) using an ion exchanger catalyst.
WO 07/050,580 discloses a preparation method of pressure sensitive adhesive compositions having acrylate/methacrylate at the polymer chain ends using chain terminators (to control molecular weight of prepared products) in a group of silanes, silazanes, disilazanes, and disiloxanes having acrylate/methacrylate at the molecular ends.